The present invention relates to the field of reflective liquid crystal display devices.
Liquid crystal display devices (LCDs) are used in a variety of appliances including display for mobile personal digital assistants, taking the advantage of the thin and light features. An LCD is a light receiving device which does not emit light itself but changes the light transmittance for displaying information. Since the LCD can be driven with a few volts, a reflective LCD, in which a reflector is provided underneath the LCD to display information using reflected external light, realizes an extremely low power consuming display device.
A conventional reflective color LCD includes a liquid crystal cell provided with a color filter and a pair of polarizing films interposing the liquid crystal cell. The color filter is disposed on one of substrates of the liquid crystal cell, and a transparent electrode is formed on the color filter. The voltage applied to this liquid crystal cell changes the ordering direction or orientation of the liquid crystal molecules, and thus changes the light transmittance of the liquid crystal for each color filter to display colored information.
The transmittance of the polarized light parallel to an absorption axis of the polarizing film is almost 0%, and that of the vertically polarized light is almost 90%. Light constituents vertical to the absorption axis in the non-polarized natural light is 50% of the total light. Accordingly, overall reflectance in the reflective LCD using two polarizing films in which the light passes through the polarizing films four times before exiting the reflective LCD is as follows when absorption of the light by the color filter and loss on the reflecting face are not considered:
(0.9)4xc3x9750%=32.8%.
The reflectance is thus limited to around 33% even for a black and white panel.
In order to achieve brighter display, several prior arts disclose configuration to employ only one polarizing film on an upper side of the liquid crystal cell, and interpose the liquid crystal cell between one polarizing film and a reflector (e.g. Japanese Laid-open Patent Nos. H7-146469 and H7-84252). In this case, the light passes through the polarizing film only twice, and overall reflectance is as follows when absorption of the light by the color filter and loss on the reflecting face are not considered:
(0.9)2xc3x9750%=40.5%.
The overall reflectance improves by about 23.5% at the maximum (=(40.5/32.8)xc3x97100%xe2x88x92100%), compared to the configuration using two polarizing films.
Color LCDs which do not employ the color filter are disclosed in the Japanese Laid-open Patent Nos. H6-308481, H6-175125, and H6-301006. The Japanese Laid-open Patent No. H6-308481 discloses the reflective color LCD which uses birefringence of a twisted nematic liquid crystal layer and a polarizing film for color display. The Japanese Laid-open Patent Nos. H6-175125 and H6-301006 propose the color LCD which uses birefringence of the liquid crystal layer and a phase retardation film for color display.
However, the reflective LCD using two polarizing films may not be able to secure reflectance for achieving sufficient brightness.
The reflective LCD using one polarizing film displays color information by the use of the color filter, and secures sufficient brightness by increasing the reflectance. This configuration, however, makes achromatic display of black and white difficult. In particular, achromatic black color which has low reflectance may not be displayed.
The reflective LCD using birefringence of twisted nematic liquid crystal layer and polarizing film for color display, and the color LCD using birefringence of the liquid crystal layer and a retardation film do not use the color filter. Since these types of color LCDs eliminate the use of the color filter, reflectance for sufficient brightness is securable even if two polarizing films are used. However, since the display is colored by birefringence, multi gray levels and multi-color display such as 4096 colors in 16-step gradation or full color in 64-step gradation may theoretically be difficult. Color purity and color reproducibility range may also be narrow.
The reflective LCD in the black and white mode which uses two polarizing films may not be able to achieve high reflectance for the white mode.
The present invention aims to offer a reflective liquid crystal display device (LCD) which achieves bright white display, high contrast, and achromatic black and white display.
The reflective LCD of the present invention includes a liquid crystal cell in which a nematic liquid crystal layer is sealed between first and second substrates; a polarizing film disposed on the first substrate side of the liquid crystal cell; two retardation films consisting of a structural component having small chromatic dispersion in refractive index anisotropy disposed between the polarizing film and liquid crystal cell; and optical reflecting means disposed on the second substrate side.
A twisting angle of the nematic liquid crystal layer is from 45xc2x0 to 90xc2x0, and a product of birefringence xcex94nLC of the nematic liquid crystal layer and thickness dLC of the liquid crystal layer, xcex94nLCxe2x88x92dLC, is from 0.20 to 0.30 Mm. The retardation value RF1 of the retardation film at the polarizing film side (a product of refractive index anisotropy and thickness of the retardation film) is from 0.23 xcexcm to 0.28 xcexcm. The retardation value RF2 of the retardation film at the liquid crystal cell side is from 0.13 xcexcm to 0.18 xcexcm. The direction normal to the film face of the two retardation films is determined as the z axis, and the direction of a slow axis is determined as the x axis in orthogonal coordinates (x, y, z). When a z coefficient Qz defined by Formula 1, using refractive indexes nx, ny, and nz to each axis direction in the above orthogonal coordinates, is from 0.3 to 1.0; a set of Formulae 2 to 4, or a set of Formula 5 to 7 is satisfied:
Qz=(nxxe2x88x92nz)/(nxxe2x88x92ny)xe2x80x83xe2x80x83(1);
75xc2x0xe2x89xa6xcfx86Pxe2x89xa695xc2x0xe2x80x83xe2x80x83(2);
95xc2x0xe2x89xa6xcfx86Pxe2x88x92xcfx86F1xe2x89xa6115xc2x0xe2x80x83xe2x80x83(3)
155xc2x0xe2x89xa6xcfx86Pxe2x88x92xcfx86F2xe2x89xa6175xc2x0xe2x80x83xe2x80x83(4);
xe2x88x9215xc2x0xe2x89xa6xcfx86Pxe2x89xa6105xc2x0xe2x80x83xe2x80x83(5);
xe2x88x92115xc2x0xe2x89xa6xcfx86Pxe2x88x92xcfx86F1xe2x89xa6xe2x88x92105xc2x0xe2x80x83xe2x80x83(6);
xe2x88x92175xc2x0xe2x89xa6xcfx86Pxe2x88x92xcfx86F2xe2x89xa6xe2x88x92165xc2x0xe2x80x83xe2x80x83(7);
where
xcfx86P=angle of the absorption axis direction of the polarizing film;
xcfx86F1=angle of the slow axis direction of the retardation film on the polarizing film side; and
xcfx86F2=angle of the slow axis direction of the retardation film on the liquid crystal cell side.
All angles are measured relative to a reference line which is a bisector of a larger angle between the ordering direction of liquid crystal molecules closest to the first substrate and the ordering direction of liquid crystal molecules closest to the second substrate. A twisting direction of the nematic liquid crystal layer from the first substrate to second substrate is determined as a positive direction.
With this configuration, the reflective LCD of the present invention in the normally white mode achieves bright display and achromatic color change between back and white.
In particular, when the set of Formulae 2 to 4 is satisfied, it is preferable to set the angle xcfx86P of the absorption axis direction of the polarizing film from 90xc2x0 to 120xc2x0 or from 155xc2x0 to 185xc2x0. This further achieves better characteristics with high contrast.
When the set of Formulae 5 to 7 is satisfied, it is preferable to set the angle xcfx86P of the absorption axis direction of the polarizing film from 0xc2x0 to 30xc2x0 or from 60xc2x0 to 90xc2x0. This also achieves better characteristics with high contrast.
Furthermore, the reflective LCD of the present invention preferably sets the twisting angle of the nematic liquid crystal layer from 60xc2x0 to 65xc2x0.
This further achieves better characteristics.
The z coefficient Qz of the retardation film at the polarizing film side in the reflective LCD of the present invention is preferably set from 0.3 to 0.7. This enables the achievement of the reflective LCD with less change in reflectance by viewing angles. From the same viewpoint, it is still preferable to set the z coefficient Qz for each of the two retardation films from 0.3 to 0.7.
The reflective LCD of the present invention achieves bright display by collecting external light around the panel with provision of a scattering film on the first substrate side. This scattering film is preferably disposed between the retardation film and first substrate in order to suppress blurring of display images. In addition, a forward-scattering film is preferably used as the scattering film. As for the forward-scattering film, it is preferable to use a material which has strong forward-scattering characteristics with almost no backward-scattering characteristics.
In the reflective LCD of the present invention, the optical reflecting means preferably contains a metal at least selected from aluminum and silver. Preferably, the metal electrode also fimctions as an electrode on the second substrate side.
This metal electrode, particularly in case of LCDs having aforementioned scattering film, preferably has a mirror-finished surface. This enables to reduce disorder in the ordering direction of liquid crystal to achieve natural visual recognition. On the other hand, in case of reflective LCDs which do not use the scattering film, it is preferable to dispose a scattering layer on the metal electrode or add diffusing reflectivity to the metal electrode itself. To add diffusing reflectivity to the metal electrode, its surface may preferably be roughened to achieve an average tilt angle of 3xc2x0 to 12xc2x0. This enables the achievement of the reflective LCD with natural visual recognition.
The reflective LCD of the present invention may also employ a transparent substrate for the second substrate, and dispose the optical reflecting means such as a diffusing reflector outside of the transparent substrate. In this case, a transparent electrode is also used for the second substrate. In this configuration, an air layer is preferably created between the transparent substrate and diffusing reflector. This further increases the diffusing effect.
Also in the reflective LCD of the present invention, a color filter is disposed to configure the reflective color LCD, or without color filter to configure the reflective LCD in the black and white mode. In the black and white mode, further bright display is achievable particularly with high reflectance for white. In the color mode, for example, full color 64-step gradation is achievable with the characteristics of achromatic color change between black and white. Provision of a nonlinear device at the second substrate side enables to further achieve an active matrix reflective LCD driven by the nonlinear device such as TFT disposed in matrix. In this case, an insulative flattening film is formed on the nonlinear device, and the nonlinear device and the electrode at the second substrate side are electrically connected through a contact hole created on this flattening film. This enables the achievement of the reflective LCD with high reflectance and high aperture ratio which can be driven actively.